The thrust needle roller bearing is formed generally by needle rollers, cages and raceway rings. These bearings have high load capacity and high rigidity nevertheless of its small projection area of bearing since it has a structure of line contact between the needle roller and the raceway ring. Accordingly, it is suited for bearing uses under severe conditions such as lean lubricant and high speed rotation. Thus, they are widely used as bearings for vehicle automatic transmissions and air compressors.
A thrust roller bearing is known which intends to increase the lubricant flow rate per unit time passing therethrough by improving at least one of the flow-in and flow-out abilities of lubricant (see Japanese Laid-open Patent Publication No. 70872/2002). FIG. 12(a) is a partial plan view showing a thrust roller bearing 50 of the prior art. FIG. 12(b) is a cross-sectional view along a line XIIb-XIIb in FIG. 12(a). FIG. 12(c) is a cross-sectional view along a line XIIc-XIIc in FIG. 12(b). The thrust needle roller bearing 50 has a plurality of needle rollers 80 and two annular cages 60 and 70. Each of the cages 60 and 70 has a plurality of windows 61 and 71 longer than a length “l” of the rollers 80. Roller holding portions 64 and 74 are formed in the windows 61 and 74 and sandwich the needle rollers 80 therebetween. The radial length “la” of each of the roller holding portions 64 and 74 of the cages 60 and 70 is formed shorter than the height “I” of the rollers 80. The two cages 60 and 70 are assembled by folding them as a unit. Thus, at least one of the portions of the unit on either radially outer and inner side of the roller holding portions 64 and 74 have their axial thickness “t1” and “t2” smaller than that “t0” of the roller holding portions 64 and 74.
The reduced thickness of the radially outer and inner side of the roller holding portions 64 and 74 makes it possible to improve the flow-in ability or the flow-out ability of the lubricant on at least one side of the roller holding portions. Thus, this increases the lubricant flow rate per a unit time passing through the bearing.
However, in the thrust needle roller bearing 50 of the prior art, the two cages 60 and 70 are assembled by superimposing radially outward flat portions 62 and 72 upon one another. The radially innermost ends 67 and 77 of the radially inner flat portions 63 and 73 are folded. Finally, the radially innermost end 67 is caulked onto the other radially innermost end 77. Such a structure permits easy separation of the superimposed flat portions 62 and 72. This causes undue stress in the caulked portion and therefore lowers or diminishes the holding ability of the needle rollers 80.
In addition, it is impossible to obtain a wide contact area between the needle bearings 80 and the windows 61 and 71 of the cages 60 and 70 at the caulking side of the radially inward flat portions 63 and 73. Thus, a drilling abrasion or an abnormal abrasion is caused in the windows 61 and 71 by an urging force of the needle rollers 80 against the wall surface of the windows 61 and 71 by the centrifugal force. This is in addition to the abrasion caused by the self-rotation of the rollers 80.
Furthermore, since two cages 60 and 70 are secured only by folding the radially innermost end 67 and by caulking it onto the other radially innermost end 77, the securing force therebetween is influenced by the dimension of the end surface. Thus, the dispersion of flatness is diminished especially at the radially innermost end 67.